MUHAMMAD ARBAB REHAN MUHAMMAD ARBAB Licentiate Thesis Production Technology 2017 No. 18 Microstructure and mechanical MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A 5 WT.% CR COLD WORK TOOL STEEL - INFLUENCE OF HEAT TREATMENT PROCEDURE. TREATMENT OF HEAT - INFLUENCE STEEL TOOL CR COLD WORK OF A 5 WT.% PROPERTIES AND MECHANICAL MICROSTRUCTURE properties of a 5 wt.% Cr cold work tool steel - Influence of heat treatment procedure. Muhammad Arbab Rehan 2017 NO.18 2017 ISBN 978-91-87531-56-9 (Printed) ISBN 978-91-87531-55-2 (Electronic) Tryck: Ineko AB, april 2017. Licentiate Thesis Production Technology 2017 No. 18 Microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel - Influence of heat treatment procedure. Muhammad Arbab Rehan University West SE-46186 Trollhättan Sweden +46-520 22 30 00 www.hv.se © Muhammad Arbab Rehan 2017 Print Book ISBN 978-91-87531-56-9 eBook ISBN 978-91-87531-55-2 Acknowledgements I must admit that there are a lot of people who have helped me with the passage of time. However, I would like to acknowledge and express my gratitude to the most important ones, my parents. They have always been understanding, helpful and stood by me in all parts of life. Their positive approach and wise suggestions have always lifted me up. I would also like to thank my wife (Mehroze Nasim Khan) who has been supportive in all aspects of life. It has been a wonderful experience growing with her together with my beautiful daughter. Moreover, my sister and brothers who had given me a joyous childhood and have always contributed to my personality. I would like to thank my colleagues in Material Science Department at Uddeholms AB for such a friendly environment especially during coffee breaks. Moreover, my industrial supervisors Berne Högman and Anna Medvedeva have always been supportive in listening to my ideas with patience and attention. Their advices are always valuable. Thank you for providing the support and help whenever I needed it. I would also like to thank Lars Göran Nord, Anders Thuvander, Per Erik Skogholm, Sebastian Ejnermark and Seshendra Karamchedu, who always have time for me for a discussion. Finally, I would like to thank my academic supervisors Prof. Lars-Erik Svensson and Prof. Leif Karlsson. I have realized that it requires an immense patience to learn and teach someone. Without their guidance and support this work could not have been possible. I look forward to learn more from them. I would also like to thank Uddeholms AB and K.K foundation for funding the research school of SiCoMap of which this industrial PhD is a part. M. Arbab Rehan 11th of May 2017 iii Populärvetenskaplig Sammanfattning Titel: Mikrostrucktur och mekaniska egenskaper hos ett kallarbetsstål med 5% Cr- Inverkan av värmebehandling. Nyckelord: Verktygstål; Austenitisering; Restaustenit; Värmebehandling; Mikrostruktur; Mekaniska egenskaper Behovet av avancerade höghållfasta stål (AHSS) inom fordonsindustrin ökar dag för dag. Det motiveras främst av det faktum att AHSS kan användas som tunna plåtar och samtidigt ha hög hållfasthet. Detta gör det möjligt att reducera vikten hos bilar. Följaktligen ökar deras bränsleeffektivitet, vilket är positivt för miljön. Det förväntas också att AHSS inom en snar framtid kommer att ha ännu högre hållfasthet. Verktygsstål med 5 vikts% Cr för kallbearbetning används ofta för att forma AHSS. Därför måste verktygsindustrin också följa med för att möta de utmaningar som AHSS ger i framtiden, det vill säga verktygsstålet måste ha en högre hårdhet, bättre tryckhållfasthet och ökad seghet. Ett sätt att uppnå målet är att utveckla en djupare förståelse för värmebehandlingen. Detta kan bidra till att förbättra de mekaniska egenskaperna hos verktygsstålet genom att modifiera processparametrarna vid värmebehandling av kallarbetsstål med 5 vikts% Cr. Det konstaterades att en högre austenitiseringstemperatur kan användas för att åstadkomma en högre hårdhet, god tryckhållfasthet och tillräckligt seghet hos stålet. Emellertid kan en för hög austenitiseringstemperatur resultera i för kraftig förgrovning av de tidigare austenitkornen, vilket resulterar i låg slagseghet. Det konstaterades också att restaustenit kan omvandlas under härdning i två olika processer. För det första, anlöpning vid 525°C resulterade i omvandling av kvarvarande austenit till martensit vid kylning. För det andra, vid anlöpning vid 600°C under lång tid kan isotermisk omvandling av restaustenit till ferrit och karbider ske. Dessa resultat användes för att förstå standardförfarandet för anlöpning av kallarbetsstål med 5 vikts.% Cr. Vidare diskuteras alternativa värmebehandlingsförfaranden med utgångspunkt från de viktigaste resultaten i denna avhandling. v Abstract Title: Microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel - Influence of heat treatment procedure. Keywords: Cold work tool steel; Heat treatment; Microstructural characterisation; Mechanical properties; Retained austenite. Print Book ISBN: 978-91-87531-56-9 eBook ISBN: 978-91-87531-55-2 The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughness of the steel. However, too high austenitisation temperature may result in excessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to martensite on cooling. Secondly, when tempering at 600°C for extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides. These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis. vii Appended Publications Paper A. Effect of austenitisation temperature and multiple tempering on the microstructure and impact toughness of a 5 wt.% Cr cold work tool steel. M.A. Rehan, A. Medvedeva, L.E. Svensson and L. Karlsson, Proceedings of the 10th International Tooling Conference, Bratislava, Oct. 2016, 91-100. I was the main author, designed the experiments and performed microscopy and thermodynamical calculations. Co-authors were involved in discussions of results and helped writing the manuscript. Paper B. Effect of austenitisation and tempering on the microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel. M.A. Rehan, A. Medvedeva, B. Högman, L.E. Svensson and L. Karlsson, Steel Research Int., Vol. 87, Issue 12, Dec. 2016, 1609-1618. Microscopy and thermodynamical calculations were performed by me. I was the main author of the manuscript. Co-authors contributed in designing the experiments, discussing the results and in writing of the manuscript. Paper C. Retained austenite transformation during heat treatment of a 5 wt.% Cr cold work tool steel. M.A. Rehan, A. Medvedeva, L.E. Svensson and L. Karlsson. Submitted to ‘Metallurgical and Materials Transactions A’. I was the main author, designed the experiments, performed microscopy and thermodynamical calculations. Co-authors were involved in discussions of the results and in writing of the manuscript. ix Table of Contents Acknowledgements ......................................................................... iii Populärvetenskaplig Sammanfattning .............................................. v Abstract ......................................................................................... vii Appended Publications ................................................................... ix Table of Contents ........................................................................... xi 1 Introduction ..................................................... 15 1.1 Background ........................................................................ 16 1.2 Aim and approach .............................................................. 17 1.3 Research questions ........................................................... 18 2 Tool steels ....................................................... 19 2.1 Cold work tool steels .........................................................